1. Field of the Invention
This invention relates to an aluminum alloy support material for a lithographic printing plate and a process for producing a support for a lithographic printing plate. More particularly it is concerned with an aluminum alloy support material for a lithographic printing plate which is highly adaptable to electrochemical surface graining and a process for producing a support having improved strength with a reduced material cost.
2. Description of the Prior Art
Lithography has generally been carried out by using an aluminum alloy sheet as a support. The surface of the sheet is roughened by surface graining treatment in order to ensure good adhesion to a photosensitive layer which is formed on the sheet and improve water retention in non-image areas.
In the past, surface graining was performed mechanically, such as by ball graining, brush graining, or wire graining, but recently an electrochemical or electrolytic graining method has been frequently employed to roughen the surface of a support. In the electrochemcial method either an aqueous electrolytic solution containing hydrochloric acid as the sole or primary electrolyte (hereinafter referred to as a hydrochloric acid-based electrolytic solution) or an aqueous electrolytic solution containing nitric acid as the sole or primary electrolyte (hereinafter referred to as a nitric acid-based electrolytic solution) is used. The electrolytic graining method is capable of providing a printing plate having excellent processability and printability, and it is greatly adapted to continuous process using a coiled strip of an aluminum alloy.
The aluminum alloy conventionally used for producing a support for a lithographic printing plate includes those corresponding to JIS A1100 (Al purity: at least 99.0% by weight) or JIS A3003 (Al purity: 98.0-98.5% by weight) for mechanical graining, and those corresponding to JIS A1050 (Al purity: at least 99.5% by weight) for electrolytic graining, from which a uniform electrolytically grained surface can be obtained.
Although A1050-type aluminum alloys give good results by electrolytic graining, they have some drawbacks due to the high purity of aluminum. First, their strength is low and hence, if they are used in the form of a thin sheet, it is very difficult to handle the printing plate produced from the sheet. Second, the printing plate tends to soften during burning treatment to make handling of the plate even more difficult.
Printing speeds have been increasing as printing technology advances. Therefore, the stress applied to the printing plate which is mechanically secured to the opposite circumferential ends of the plate cylinder of a printing press is increased. If the support for the printing plate does not have sufficient strength, the secured ends of the plate will be deformed or broken, causing troubles such as printing shear or breakage of the printing plate, which often makes it impossible to continue the printing operation.
Accordingly, in order to assure that the plate has sufficient mechanical strength and dimensional stability, it is necessary to use a relatively thick aluminum alloy sheet as a support. This is a main cause of the high cost of lithographic printing plates.
Thus, aluminum alloys having a purity of at least 99.0% and particularly at least 99.5% by weight of Al such as JIS A1050-type aluminum alloys are excellent in adaptability to electrochemical graining, but their strength is not sufficient.
On the other hand, aluminum alloys containing less than 99.0% by weight aluminum and particularly JIS A3003-type aluminum alloys have high strength, but they give poor results (non-uniformly grained surface) when subjected to electrochemical graining. In addition, they have the disadvantage that tinting or staining in non-image areas tends to occur during printing.
Japanese Patent Application OPI (KOKAI) No. 60-63340 (1985) discloses that the uniformity of an electrochemically grained surface can be improved by controlling the number of eutectic intermetallics crystallized in an Al-Mn-Fe alloy per unit area of the sheet. However, even this technique cannot provide a uniformly grained surface by electrolysis.
If the grain formed on the surface of a support by electrochemical graining is not sufficiently uniform, the printing endurance limit or the maximum number of clear printings that can be obtained by a printing plate will decrease.
U.S. Pat. No. 4,686,083 discloses an aluminum alloy support for a lithographic printing plate which is suitable for electrochemical graining and which has improved fatigue resistance, heat softening resistance, and printability. The aluminum alloy disclosed therein comprises 0.05 to less than 0.8% Mn, at most 0.2% Si, at most 0.5% Fe, at most 0.1% Ti, and at most 0.02% B on a weight basis. A similar electrolytically grained aluminum alloy support which contains at least 0.2% Si is proposed in U.S. patent application Ser. No. 361,430.
Japanese Patent Application OPI (KOKAI) No. 52-39403 (1977) discloses an Al-Mn alloy support which contains 0.8-1.5% Mn, less than 1.5% impurities, and optionally less than 2.5% Mg on a weight basis and which needs no graining treatment.
Other aluminum alloy support materials are disclosed in U.S. Pat. Nos. 3,944,439; 4,427,500; 4,301,229; 4,435,230; 4,547,274; Japanese Patent Application OPI (KOKAI) No. 60-63347 (1985); Japanese Patent Publications (KOKOKU) Nos. 1-46577 (1989) and 1-47544 (1989).